Transformation of Tom Riddle to Lord Voldemort

Dumbledore took the diary from Harry and peered keenly down his long, crooked nose at its burnt and soggy pages.
'Brilliant,' he said softly. 'Of course, he was probably the most brilliant student Hogwarts has ever seen.' He turned around to the Weasleys, who were looking utterly bewieldered.
'Very few people know that Lord Voldemort was once called Tom Riddle. I taught him myself, fifty years ago, at Hogwarts. He disappeared after leaving the school... traveled far and wide... sank so deeply into the Dark Arts, consorted with the very worst of our kind, underwent so many dangerous, magical transformations, that when he resurfaced as Lord Voldemort, he was barely recognisable. Hardly anyone connected Lord Voldemort with the clever, handsome boy who was once Head Boy here.'

DOBBY'S REWARD - Pg 347